Thermionic reactor electrical control system

ABSTRACT

A thermionic reactor electrical hookup and control system which provides for regulation of plant electrical output, individual control of fuel element emitter temperatures, and isolation of fuel element failure events. The control system is applicable to fast and thermal spectrum thermionic reactors for ground, sea and space applications.

United States Patent 11 1 [111 3,801,445

Wilkins et al. Apr. 2, 1974 [54] THERMIONIC REACTOR ELECTRICAL 3,234,4122/1966 Sankovich et al. 176/39 X CONTROL SYSTEM 3,493,792 2/1970Antermyer 3,259,766 7/1966 Beckjord et a]. 176/39 [75] Inventors: DanielR. Wilkins, r g P l R- 3,673,440 6/1972 Paine et al 310 4 Hill, SanJose, both of Calif. 3,607,631 9/1971 Hobson 310/4 [73] Assignee: TheUnited States of America as represented by the United States PrimaryExaminer-Ben amin R. Padgett Atomic Energy Commission AssistantExaminer-P. K. Pavey Washington DC Attorney, Agent, or Firm-John A.Horan; F. A. Robertson; L. E. Carnahan [22] Filed: Sept. 28, 1972 [21]Appl. No.: 293,175 [57] ABSTRACT A thermionic reactor electrical hookupand control [52] US. Cl,,, 176/39, 176/20 R, 310/4 R system whichprovides for regulation of plant electri- [51] Int. Cl G2lc 7/00 Cal pindividual control of fuel element emitter [58] Field of Search 310/4 R;176/20 R, 39 temperatures, and isolation of fuel element failure events.The control system is applicable to fast and [56] Referen e Cit dthermal spectrum thermionic reactors for ground, sea

UNITED STATES PATENTS and Space applicatims- 3,113,091 12/1963 Rasor eta] 176/39 X 6 Claims, 3 Drawing Figures c HTR 54 5 CONTROL LOAD 5O POWERREGULATION MODULE fl/ P38 24 *26 IO TFE CURRENT DC CONTROL Bus R v E asL L 42 E T REACTOR BALLAST g est/ ast l7 FLUX SENSOR 33 I LOAD 4| 32CONTROL PATENIEWR 2:914

sum 10F 2 Cs HTR LOA CONTROL D 50 23 POWER REGULATION TFE CURRENT Dc TR|6 CON 0L BUS 1w 3 fnl N l5 ATOR R v E f L F 35 42 L E c REACTOR gFILTER POWER BALLAST R CONTROL LOAD 4? FLUX SENSOR 33 I9 \I BALLAST 8LOAD 4| 32 CONTROL Fig. 1

F fg. 2

SHEET 2 0F 2 FULL POWER RELATIVE THERMAL POWER FULL POWER "MENIEnA'R 2m4 BACKGROUND OF THE INVENTION The invention described herein was madein the course of, or under, Contract No. AT(04-3)-77l, with the UnitedStates Atomic Energy Commission.

This invention relates to thermionic reactors, particularly to a controlsystem for thermionic reactors, and more particularly to an electricalcontrol system capable of utilizing two methods for regulating athermionic reactor either separately or in conjunction with one another.

Thermionic generators useful for remote electrical power sources areknown in the prior art as evidenced by U. S. Pat. Nos. 3,272,658 issuedSept. 13, 1 966 and 3,296,032 issued Jan. 3, 1967. Control systems forthermionic converters are also known in the prior art as exemplified byU. S. Pat. Nos. 3,493,792 issued Feb. 3, 1970, and 3,532,960 issued Oct.6, 1970. In addition much prior effort has been directed toward thedevelopment of a thermionic reactor for utilization in spaceapplications. While these prior efforts have produced satisfactoryresults there exists a need for a simple yet effective control systemfor regulation of plant electrical output and control of the fuelelements thereof.

SUMMARY OF THE INVENTION The present invention provides an electricalhookup and control system for a thermionic reactor which provides forregulation of plant electrical output, individual control of fuelelement emitter temperatures, and isolation of fuel element failureevents, thereby fulfilling the above-mentioned need for a simple yeteffective control system.

Therefore, it is an object of the invention to provide an electricalcontrol system for a thermionic reactor.

A further object of the invention is to provide an electrical controlsystem capable of utilizing two methods of regulating a thermionic powerplant which methods may be utilized individually or in combination.

Another object of the invention is to provide an electrical controlsystem which involves either the utilization of a ballast load on a d-cbus and/or in response to error signals from the do bus for regulating athermionic power plant.

Another object of the invention is to provide a thermionic reactorelectrical hookup and control system for regulation of plant electricaloutput, individual control of fuel element emitter temperatures, andisolation of fuel element failure events.

Another object of the invention is to provide for an electrical controlsystem applicable to fast and thermal spectrum thermionic reactors forground, sea and space applications.

Other objects of the invention will become readily apparent from thefollowing description and accompanying drawings:

BRIEF DESCRIPTION OF THE INVENTION FIG. 1 diagrammatically illustratesthe inventive electrical control system;

FIG. 2 is a graphical illustration of the steady state relationshipbetween current and thermal power for the control law utilized in thecontrol system; and

FIG. 3 graphically shows the emitter temperatures as a function ofthermal power when utilizing the inventive system.

DESCRIPTION OF THE INVENTION The invention is directed to a thermionicreactor electrical hookup and control system which provides forregulation of plant electrical output, individual control of fuelelement emitter temperatures and isolation of fuel element failureevents. The inventive control system utilizes two methods for regulatingthe thermionic power plant which may be utilized singly or incombination, these being the use of a ballast load on a d-c bus, and byresponse to error signals from the d-c bus.

The inventive electrical control system is diagrammatically illustratedin FIG. 1 wherein a reactor vessel 10 is provided with thermionic fuelelements (TFE) connected electrically in pairs, only two such TF Eindicated at 11 and 12, are shown for clarity, one TFE of each pairbeing connected positive and one negative with respect to ground. By wayof example, a reactor core may contain about pairs of twelve cell TFEsand may additionally be provided with an outer row of 48 beryllium oxideinternal reflector rods (not shown). Also, for example, the TFEs are1.01 inches in diameter with an outer sheath material of Ceramvar andare provided with a handle assembly, indicated generally at 13, forhandling operations and electrical and mechanical connections, suchhandle assembly, for example, containing a cesium reservoir 14 (only oneshown) and a fission product trap (not shown). Coolant, such as NaK, iscirculated through vessel 10 and upwardly between the TFEs, as indicatedby the arrows, and is connected by inlet 15 and outlet 16 withappropriate heat exchangers, not shown. An external reflector cupassembly 17 is positioned about the lower portion of vessel 10 andprovides the reactivity control of the TFEs, along with the internalreflector rods, reflector cup 17 being raised and lowered by an actuator18 which may be manually controlled by means not shown, but known in theart as well as by an electrical reactor power control 19 connectedelectrically therewith via lead 30. Inasmuch as the details of the TFEs11 and 12, the reactor vessel 10 and the reflector cup 17 and actuator18 do not constitute part of this invention, further description thereofis deemed unnecessary.

The following description of the electrical control system is directedto a single pair of TFEs but it is understood that each pair of TFEs inthe reactor vessel will be controlled in a like manner by the inventivesystem. Referring now to the control for the pair of TFEs l1 and 12electrical current leads indicated at 21 and 22, respectively, are fedto a power regulation module 23, the output terminals thereof beingconnected in parallel by electrical leads 24 and 25 on a d-c busindicated generally at 26 containing an electrical load 27. The powerregulation module 23 serves the following two important functions:

1. To control the electrical current extracted from each TFE pair inaccordance with a control law to be described subsequently.

2. To perform a reliability function in the event that a partial orcomplete open circuit failure is encountered in either TFE of a TFEpair.

The TFE pair current, I in lead 22, indicated at 28 and directed to anelectronic adder 29, is controlled to a demand signal, I;*, in an outputlead 30 from a signal generator 31. The demand signal I,* is linearlyrelated to the measured thermal power level (P of the reactor which issensed by a flux sensor 32 and transmitted through a filter 33 to thesignal generator as indicated by leads 34 and 35. An error signal 36proportional to the difference I,-I is generated in the electronic adder29, and directed to a TFE current control 37, the output therefromindicated at 38 is fed into power regulation module 23, which asdescribed hereinafter regulates the electrical current extracted fromeach TFE pair, the first of the above-mentioned functions.

Regarding the second of the above-mentioned functions, electronicswitches (not shown) are located with each power regulation module 23,between each TFE current leads (21-22) and ground. One of these switchescan be closed in the event that an open circuit failure is encounteredin either TFE 11 or 12 to convert the open circuit to a short circuitand permit the power from the unfailed TFE to continue to be processed.

A third function of the power regulation module 23 is to step up the d-cvoltage level to a value which is consistent with the demands of thepower user (load 27).

As described above, output terminals 24 and 25 of power conditioningmodule 23 are connected in parallel on d-c bus 26 which contains anelectrical load 27. In addition, the d-c bus contains a ballast load 40and control circuit therefor which includes an electronic adder 41connected to receive a voltage load signal (V from d-c bus 26 asindicated at 42 and to receive a voltage control or demand signal 43,electronic adder 41 producing an error signal 45 proportional to thedifference between the actual d-c bus voltage V (42) and the demandsignal (43) which is directed to a ballast load control 46, the outputof which, as indicated by lead 47, is directed to ballast load 40 forcontrolling same.

Automatic control of the temperature of the TFE cesium reservoir 14 isobtained by a heater such as electric coils 48 wrapped about reservoir14 and connected via leads 49 to a heater control 50 activated by anerror signal 51 from an electronic adder 52 which is proportional to thedifference between the actual temperature (T,,) from the reservoir 14indicated at 53 and desired or demand temperature value T f" indicatedat 54.

Control of TFE pair of electrical currents is based upon the fact that,for a given emitter temperature, electron cooling in a thermionic cellis proportional to current over a broad current range; and is onlyweakly influenced by variations of electrode work functions, cesiumreservoir temperature and interelectrode spacing. The herein selectedcontrol law, calls for controlling the TFE pair electrical current at ornear open circuit until a threshold reactor thermal power level isreached. This threshold is selected such that TFE emitters will haveachieved operating temperature but will be rejecting heat to thecollectors through radiation, cesium conduction and structural lossesonly. Once this threshold reactor thermal power level is reached,additional increases in reactor thermal power will be accompanied byproportionate increases in TFE pair electrical current to balanceelectron cooling heat transfer against the increased power generation.In this manner, emitter temperatures of thermionic fuel elements areheld approximately constant over a range of electrical loads fromeffectively zero to 100 percent.

The steady state relationship between current and thermal power for thiscontrol law is depicted in FIG. 2; while emitter temperature as afunction of thermal power is shown in FIG. 3. TFEs in different radialzones of the reactor may be operated at the same emitter temperature byvarying the thermal power threshold above which electrical current isdrawn and the proportionality constant relating current to reactor powerlevel.

Automatic set point control of TFE cesium reservoir temperatures isutilized in the herein described thermionic reactor. TFE cesiumreservoir temperatures will normally be established prior to or duringthe plant startup sequence and not be changed during normal plantoperation. This straightforward cesium reservoir temperature controlarrangement is facilitated by operating the TFEs at fixed emittertemperatures.

Two methods for regulating the d-c bus voltage are carried out in theinventive control system for the thermionic power plant:

The first method involves the use of ballast load 40 on d-c bus 26wherein the ballast load is varied via ballast load control 46 tomaintain the d-c bus voltage constant. This control provides a preciseregulation of bus voltage over a range of power levels dictated by theca pacity of the ballast load.

The second method consists of controlling reactor thermal power level inresponse to an error signal from the d-c bus 26. In this controlprocedure, if the bus voltage falls below tolerance as indicated by thevoltage load (V error signal indicated at 45' and directed to reactorpower control 19, reactivity is inserted to raise the thermal powerlevel of the reactor, this being accomplished by a command signaltransmitted via lead 20 to actuator 18 which control the reflector 17.This increases the electrical current extracted from each TFE pair and,in turn, the bus voltage. The process continues until the error signal45, derived from the d-c bus voltage via adder 41, is reduced to zero.

The second method for regulating d-c bus voltage in a thermionic powerplant reduces or eliminates the need for ballast load 40, but thismethod of bus voltage regulation is much slower than can be accomplishedwith a ballast load, and requires more frequent movement of the controldrums. Therefore, a combination of the two control procedures mayeffectively byutilized with variations in ballast load being employedfor fast regulation of the bus voltage in response to small and/orfrequent changes in the user electrical load, and gross changes inelectrical load being accommodated on a slower basis by changing thepower level of the reactor.

The above described control of the TFB electrical currents and cesiumreservoir temperatures provides a number of operational and reliabilityfeatures outlined as follows:

1. Electrical control is modular. This feature enhances reliability, andfacilitates buildup of the reactor from a non-TFE core to a TF E core inexperimental reactors. As each TFE pair is added to the reactor vessel10, a power regulation module 23 and associated control elements isincorporated and the output is put on the d-c bus 26. Nuclear andhydraulic control of the plant are unchanged.

2. From the plant operators viewpoint, operation of the thermionic plantis similar to any plant. Specifically, the reactor operator is primarilyconcerned with thermal power, hydraulic variables and plant electricalload and is not routinely concerned with TFE electrical currents, orcesium reservoir temperatures.

3. Reduced thermal cycling of emitters is expected to prolong plantlifetime. Following a scram, the electrical currents from the TFE pairsare initially reduced at the rate required to remove stored energy fromthe fuel and emitter temperatures remain unchanged. Once the electricalcurrents reach zero (or the selected minimum), the emitters radiationcool to the collectors. This leads to a far less severe thermal shockthan will occur if full current continues to be demanded from the TFEpair even after the reactor thermal power has been reduced to zero.

4. The electrical control system is permissive of cell and TFEdegradations and failures. Variations in cesiated electrode workfunctions, interelectrode spacing and cesium pressure will have minimalinfluence on cell and TFE operating conditions. Inter-electrode shortcircuit, loss of cesium open circuit and collector short to groundfailure events, lead to a loss of output from only those cells directlyassociated with the failure event. All other cells in the reactorcontinue to operate on a business-as-usual basis. In the event that celldegradations and failures reduce the d-c bus voltage below tolerance,the power level of the reactor is raised to make up the voltagedeficiency. Hence, the effects of failure are shared by all theremaining cells in the reactor not by a few cells in the vicinity ofthose which have failed.

5. A fast reactivity feedback from core structure and coolant isaccomplished in the inventive electrical control system.

6. The inventive control system does not have a thermionic burnoutbehavior. In particular, the emitter temperature rises to the fulloperating temperature at approximately one-third design rated power, andis constant for thermal powers above this level, is illustrated in FIG.3.

It has thus been shown that the inventive electrical hookup and controlsystem provides regulation of thermionic power plant electrical output,individual control of fuel element emitter temperatures, and isolationof fuel element failure events, the control system being applicable tofast and thermal spectrum thermionic reactors for ground, sea and spaceapplications.

While a particular embodiment of the inventive electrical control systemhas been illustrated and described, modifications and changes willbecome apparent to those skilled in this art, and it is intended tocover in the appended claims, all such modifications as fall within thespirit and scope of the invention.

We claim:

1. In a thermionic reactor having a vessel containing at least one pairof thermionic fuel elements and means for controlling the reactivity ofthe reactor, a control system comprising: means for controllingtemperature of a cesium reservoir for each associated thermionic fuelelement, a d-c bus means, power regulation means electrically connectedto said d-c bus means and operatively connected to a pair of associatedthermionic fuel elements, a ballast load means connected to said d-c busmeans, control means for said ballast load means, and reactor powercontrol means electrically connected to said d-c bus means andresponsive to an error signal therefrom for activating an associatedreactor reactivity control means.

2. The control system defined in claim 1, wherein said cesium reservoirtemperature control means comprises heater means for said cesiumreservoir, heater control means operatively connected to said heatermeans, and means responsive to the temperature of said reservoir and toan established control temperature for activating said heater controlmeans.

3. The control system defined in claim I, wherein said power regulationmeans comprises a power regulation module constructed to receive currentinputs for an associated pair of thermionic fuel elements and havingoutputs connected to said d-c bus means, current control meansoperatively connected to said module means, and means responsive tocurrent inputs to said module meansand to an established control signalfor activating said current control means.

4. The control system defined in claim 3, wherein said means foractivating said current control means comprises an electronic addermeans constructed to receive current inputs from an associated pair ofthermionic fuel elements and to receive a control signal from a signalgenerator means, said signal generator means being constructed toreceive input signals representing thermal power of an associatedthermionic reactor.

5. The control system defined in claim 1, wherein said control means forsaid ballast load means comprises a ballast load control operativelyconnected to said ballast load, and an electronic adder means responsiveto signals representing voltage load on said d-c bus means and voltageload control signals for activating said ballast load control.

6. The control system defined in claim 1, additionally including anexternal load connected to said d-c bus means.

1. In a thermionic reactor having a vessel containing at least one pairof thermionic fuel elements and means for controlling the reactivity ofthe reactor, a control system comprising: means for controllingtemperature of a cesium reservoir for each associated thermionic fuelelement, a d-c bus means, power regulation means electrically connectedto said d-c bus means and operatively connected to a pair of associatedthermionic fuel elements, a ballast load means connected to said d-c busmeans, control means for said ballast load means, and reactor powercontrol means electrically connected to said d-c bus means andresponsive to an error signal therefrom for activating an associatedreactor reactivity control means.
 2. The control system defined in claim1, wherein said cesium reservoir temperature control means comprisesheater means for said cesium reservoir, heater control means operativelyconnected to said heater means, and means responsive to the temperatureof said reservoir and to an established control temperature foractivating said heater control means.
 3. The control system defined inclaim 1, wherein said power regulation means comprises a powerregulation module constructed to receive current inputs for anassociated pair of thermionic fuel elements and having outputs connectedto said d-c bus means, current control means operatively connected tosaid module means, and means responsive to current inputs to said modulemeans and to an established control signal for activating said currentcontrol means.
 4. The control system defined in claim 3, wherein saidmeans for activating said current control means comprises an electronicadder means constructed to receive current inputs from an associatedpair of thermionic fuel elements and to receive a control signal from asignal generator means, said signal generator means being constructed toreceive input signals representing thermal power of an associatedthermionic reactor.
 5. The control system defined in claim 1, whereinsaid control means for said ballast load means comprises a ballast loadcontrol operatively connected to said ballast load, and an electronicadder means responsive to signals representing voltage load on said d-cbus means and voltage load control signals for activating said ballastload control.
 6. The control system defined in claim 1, additionallyincluding an external load connected to said d-c bus means.